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Magnets
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Subject facts
Why you need to know these facts.
Vocabulary Amazing facts
Common Misconceptions
Questions
Teaching Ideas
Subject Facts
The discovery of magnets occurred in ancient times.They were known about and used by the early Greek and Chinese civilisations.The word magnet comes from Magnesia, a place in Turkey where the natural magnet lodestone can still be found.
Magnets and magnetic materials
A magnet is an object that is able to exert a strong force of attraction on materials that contain the metals iron, cobalt, steel or nickel.These are called ferromagnetic materials. Alternatively, we can describe ferromagnetic materials as materials that are susceptible to the attractive forces of magnets and have the potential to become permanent magnets themselves.
Magnets lose their magnetism permanently when they are heated beyond a specific temperature, the Curie point, because the movement of the individual atoms becomes too great to maintain a static magnetic field. All other materials are either paramagnetic (very slightly attracted by a magnetic field) or diamagnetic (very slightly repelled by a magnetic field).'Very slight' here means not detectable by anything but highly sensitive, specialised detectors.
Permanent magnets
Before 1 820, the only way to make a permanent magnet was
to stroke a steel or iron rod with a natural magnet (a piece of lodestone).The accepted model of how this works is that a lump of any magnetic material is made up of particles that are themselves 'mini-magnets'. Normally, these magnets will be pointing randomly in different directions, so their effects 'cancel out'. Stroking them with a magnet aligns them so that they all take up the same orientation, turning the magnetic material into a magnet.
Magnets produced in this way need very careful handling, as they are likely to lose their magnetism and revert to simply being magnetic material. Dropping and banging will cause the particles to become random once more as their alignment is disturbed. Materials which can be magnetised (and demagnetised) easily are known as 'soft magnetic materials'.They include nails, paper clips and an iron/silicon alloy called Stalloy, which is used in the core of electromagnets.
Since the discovery of the electromagnetic effect by Hans Christian Oersted in 1 820, it has been possible to create a permanent magnet by placing a steel rod in a coil of wire which is then attached to a strong battery.The effect of the electric coil is similar to that of the lodestone (see Chapter l, page 28).The strength of the electromagnet created depends on the number of turns in the coil and the electrical current passed through the coil.
The strongest type of permanent metallic magnet in widespread current use is an alloy of aluminium, nickel, cobalt, and copper, called alnico. An alternative is an artificial ceramic material called magnadur, which contains powdered ferromagnetic oxides. These and others are collectively known as 'hard magnetic materials'. Although they are permanent as magnets, alnico and magnadur are very brittle and prone to breakage as materials, If you keep dropping a steel magnet, you will end up with a lump of steel; if you drop a magnadur magnet, you will end up lots of tiny magnets. Alnico and magnadur magnets can easily be made into a variety of shapes, including bars and rings, for different purposes.
Most magnets found in toys and used in schools tend not to be of the steel type, because steel magnets are more likely than alloy or ceramic ones to lose their magnetism through rough handling. Often bar and horseshoe magnets, which would have been made from steel in the past, are made from plastic with correctly orientated ceramic magnets inserted in the pole areas.
This tends to make them more powerful and reliable; but it can also mean, particularly in the case of horseshoe magnets, that the magnetic field produced is not what you would get from a continuous steel magnet.
Temporary electromagnets
If a rod of a soft magnetic material (such as iron) is used instead of a steel rod in an electromagnet, the magnet will only last as long as the current is flowing: the rearrangement of the particles in the rod is only temporary.This effect has a number of applications: separating magnetic and non-magnetic materials; lifting and moving magnetic materials; operating bells and switching devices.
Magnetic poles
The poles of a magnet are the regions where the magnetic field is strongest. Magnets tend to orientate themselves in a way that corresponds to the Earth's magnetic field . A magnet can be said to have a north-seeking pole (which tends to align itself with the north magnetic pole of the Earth) and a south-seeking pole.These are usually referred to as its •north' and 'south' poles.
Magnetic forces
There is a magnetic force of attraction between a magnet and a magnetic material. However, the attractive force between two magnets is only between opposite poles: the north-seeking pole of one magnet and the south-seeking pole of another. Bringing two north-seeking or two south-seeking poles together will result in a force of repulsion between them. Placing a magnetic material between the two like poles will dissipate the repulsive force acting between them, since both poles will be attracted to the magnetic material (see Figure l).
Alnico or magnadur ring magnets can be placed over a nonmagnetic core in an alternating pattern: two north side up, two south-side up, two north-side up and so on. Each pair of rings will 'float' above the one below it, kept up by the repulsive force (see Figure 2). If the top pair are lifted and dropped, they will bounce,
Maglev
Magnetic levitation or 'maglev' can be used as a fairly frictionfree means of lifting and moving an object by means of powerful magnets. Some short-run passenger trains, such as those running between passenger terminals at certain airports like Birmingham, make use of this principle, as well as longer, faster tracks such as the Shanghai Transrapid.The train carries a set of powerful magnets on its underside, causing it to levitate when electromagnets in the track are switched on. By altering the orientation and strength of the electromagnets, the maglev train can be made to start or stop, go backwards or forwards and go faster or slower.
Lines of magnetic force
The strength of a magnetic pole reduces with distance from it. Although the force fields produced by magnets cannot be seen directly, their effects on small particles of a magnetic material (such as iron filings) can be.
NB Do not let children handle iron filings directly. The filings stick very effectively to sweaty little fingers, making them difficult to wash off. From that point, if the children rub their eyes, it can cause serious and very painful damage. Filings should be handled only by the teacher, and otherwise kept in a clear, sealed container (such as a Petri dish). Covering the poles of a magnet with paper will stop filings getting stuck to them.
The pattern created when iron filings are brought within the magnetic fields of adjacent magnets gives an indication of their attractive and repulsive force (see figure 3). Note that the lines of magnetic force run straight between adjacent north and south poles, but push away from each other between adjacent north-north or south-south pairs. Also note that the lines of force may come very close together, but they never cross. these lines can only be disrupted by other magnets or magnetic materials: the magnetic field will go through any other materials as if they didn't exist.
Why you need to know these facts
Most primary school children will have some experience of magnets. They are usually fascinated by the effects of this 'invisible force', which is why magnets often feature in toys and games. The effects of magnets can be observed and described, but there is still considerable scientific controversy over the exact nature of magnetism.
Vocabulary
Ferromagnetic material — a type of metal (iron, steel, cobalt or nickel) that can be strongly magnetised.
Magnet — an object that produces a magnetic field.
Magnetic material — a material that is affected by a magnetic field. North-seeking pole the pole of a magnet that aligns itself to the Earth's magnetic north pole.
South-seeking pole — the opposite of a north-seeking pole.
Amazing facts
The world's largest electromagnet, in Russia, contains more metal than the Eiffel Tower.
The world's fasted commercial train is the Shanghai Transrapid Maglev which has a maximum operating speed of 268mph (43 1 kmph).
Common Misconceptions
All metals are attracted to magnets.
This statement calls for some general work on properties of metals. Metals being called by their colour rather than their actual metal content can be confusing: 'copper' and 'silver' coins are really alloys of several different metals and may not be consistent between currencies (British 'silver' is non-magnetic, French 'silver' is magnetic). 'Tin cans' are not made of tin; 'silver' foil does not contain silver. A set of metal samples from an educational supplier will be a useful investment.
Bigger magnets are more powerful.
This generalisation can be challenged by testing. What would be a fair test for the strength ofa magnet? (See Teaching ideas below.) Variables to consider might be its mass, what it is made of and its shape.You will find that a small alnico magnet is much stronger than a larger piece of lodestone.The strength of a magnet depends on the extent to which the atoms in the material are aligned.
Questions
What is a magnet?
It is an object with the special property of being able to attract certain 'magnetic' metals. We can only describe a magnet by what it does — there is, as yet, no convincing simple explanation of what a magnet is.The idea that a magnet is made up of 'mini-magnet' particles that are lined up to face the same way may help some older, more imaginative children, but could be confusing to others.
Teaching ideas
Magnetic materials (exploring and sorting)
Give the children a collection of various materials, both magnetic and non-magnetic, and ask them to sort these into the two categories — first predicting, then testing with a magnet. At a later stage, this can be refined so that the children sort samples of different metals.
Exploring magnets (exploration leading to planned investigation)
Give the children a selection of magnets to explore. Once they are familiar with the basic properties of magnets, they can refine their work by setting down clear questions to investigate, such as:
Do bar magnets point north? Are the biggest magnets the strongest? Do magnets work through all materials?
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